1. Field of the Invention
The present invention relates to an information recording apparatus and method capable of radiating a light beam onto an information recording medium to record multi-level data in a beam-radiated portion.
2. Description of the Related Art
To increase capacity and processing speed of computers in the future, increases in density and capacity of computer memories have become an important technical subject. In currently available optical memories, such as optical disks and cards, only a one-level signal (i.e., a signal corresponding to the presence or absence of a record) can be written in a single record spot. In such memories, a recording density of 10.sup.8 bits/cm.sup.2 is the practical upper limit, which is not sufficient to achieve the aforestated design goals for computers.
In order to overcome this memory density limit, a multi-value signal recording system may be considered for writing a multi-value signal in a single record spot. An ultra-high density optical memory using photo-chemical hole burning (PHB) is known as one example of a memory employing such a multi-value recording system (Topical Meeting on Optical Data storage, Mar. 11-13, 1987, Stateline, Nev.). PHB is a phenomenon in which holes are generated in the absorption spectrum of a guest dispersed in a host recording medium. In this phenomenon, a photochemical reaction occurs only in a guest which has absorbed a laser beam, and the guest in an energy state at a laser wavelength position is developed to generate a hole. If "0"- and "1"-level signals based on the presence/absence of a hole are used, multi-value recording in units of wavelengths can be accomplished.
In such a multi-value recording system using PHB, however, the PHB memory film must be kept at a temperature as low as several to several ten degrees K., and many problems in using such film as an optical memory device remain unsolved. Hence, any practical application of the system has not yet been realized.